1. Field of the Invention
The present invention relates to a surface acoustic wave (SAW) filter device, in particular a surface acoustic wave filter device which is suitable for a CDMA (Code Division Multiple Access) communication systems, and also to a transducer which can be suitably used for such a surface acoustic wave filter device.
2. Description of the Related Art
With a recent progressive development in digital communication technology, there have been proposed various communication systems. For example, FDMA (Frequency Division Multiple Access) system and TDMA (Time Division Multiple Access) system are used for communication purposes, wherein frequency band or time band is divided and assigned to each station. On the other hand, the CDMA system makes use of superimposed signals in terms of frequency and time, and is recognized to be highly useful in view of the possibility of preserving a large number of channels. Thus, there is a strong demand for a further development in the CDMA system. In this connection, various requirements in terms of filtering characteristics are imposed on surface acoustic wave filter devices for the CDMA communication system, which may be summarized as follows.
(a) That the filter device has an insertion loss which is not greater than 10 dB.
(b) That the filter device has a frequency characteristic which is free from distortion over a wide range.
(c) That the filter device satisfies a T.T.E. (Triple Transient Echo) attenuation level which is not less than 30 dB.
As a surface acoustic wave filter device satisfying these requirements, one may think of using a transversal-type surface acoustic wave filter device which includes a piezoelectric substrate, as well as a bidirectional input side transducer and a bidirectional output side transducer, both formed on the substrate. That is to say, because a bidirectional transducer has a frequency characteristic which is substantially free from distortion, it would be possible to realize a wide range filter device having a frequency characteristic which is substantially free from distortion over a wide range, by combining two bidirectional transducers as the input side and output side transducers.
The surface acoustic wave filter device in which two bidirectional transducers are used as the input side and output side transducers proved to be advantageous in terms of its frequency characteristic which is substantially free from distortion over a wide range. On the other hand, however, when the surface acoustic wave filter device is constituted as a transversal-type filter device, it suffers from a significant insertion loss because surface acoustic waves excited by the input side transducer propagate in both directions. Furthermore, it is difficult to use bidirectional transducers as a filter device for CDMA communication systems because of an excessively high T.T.E. attenuation level due to a symmetrical electrode arrangement. In this instance, it would be possible to attenuate the T.T.E. level by 30 dB or more, by an impedance mismatching of the filter device. However, such an impedance mismatching would result in further increase in the insertion loss. In other words, it has been believed that a T.T.E. attenuation level of 30 dB or more cannot be achieved without impedance mismatching of the transducers and resultant increase in the insertion loss. For these grounds, there have been no surface acoustic wave filter device which can be practically used for a CDMA communication system.